Mineralogical and geochemical characteristics of near-vent metalliferous sediments: Implications for hydrothermal processes along the southern Mid-Atlantic ridge (12°S–28°S)

Abstract

In this study, we present mineralogical and geochemical analyses of 50 unconsolidated submarine surface sediments collected from separate locations within six active hydrothermal fields (Zouyu, Taij, Caifan, Deyin, Xunmei, and Tongguan) along the southern Mid-Atlantic Ridge (SMAR) at 12°S–28°S. The samples consisted of 33 metalliferous sediments (MS) and 17 non-metalliferous sediments (non-MS). The study area was divided into a northern segment that contained the Zouyu, Taij, Caifan, and Deyin hydrothermal fields and a southern segment containing the Xunmei and Tongguan hydrothermal fields. The presence of chalcopyrite, octahedral pyrite, and cubic pyrite without streaks on the surface of the sediments reflected a stable high temperature mineralization environment with a small temperature gradient in the Xunmei hydrothermal field. Cubic pyrite with growth striations co-existing with pentagonal dodecahedon-form pyrite indicated an unstable ore-forming environment with large temperature gradient in the early stage and a more stable ore-forming environment in the late stage in the Tongguan hydrothermal field. Major and trace element geochemistry data indicated that the most important material source for the abiogenic component of the sediments along SMAR 12°S–28°S was hydrothermal materials, the second most important source was lithogenic components, and the third source was elements scavenged from seawater. The Xunmei and Tongguan hydrothermal fields may have recorded phase separation, a process that would have resulted in the formation of two separate hydrothermal fluids, thereby forming two types of sulfide-rich MS with rare earth element (REE) patterns with positive and absent Eu anomalies, respectively. The Si-Fe-Mn oxide-rich MS had positive Eu anomalies, which also indicated that these oxides in sediments were directly precipitated from initial hydrothermal fluids at high temperature. The chlorite-rich MS exhibited nearly flat REE patterns similar to the volcanic material-rich samples and mid-ocean ridge basalt (MORB), which reflected the origin of the hydrothermal alteration of volcanic materials. In the northern segment, most of the MS were derived from plume fall-out Fe oxide particles. In addition, the chlorite-rich MS was likely derived from hydrothermal alteration. In the southern segment, the hydrothermal materials in the MS were primarily derived from the collapse of sulfide-bearing chimneys and Si-Fe-Mn oxide debris directly precipitated from hydrothermal fluids.